Smart combinations of different communication units, e.g., cellular mobile phones and Global Positioning System (GPS) receivers are getting more and more important since an increased number of position dependent applications (like route guiding applications, telematics running almost anywhere at any time etc.) are spreading rapidly in the market place.
One such example is a direct sampling receiver for a global positioning system that corresponds to the use of a pair of amplification and comparison units for amplifying an RF-signal received through related antennas in a diversity apparatus for a global positioning system. In the global positioning system one of the switching units is selected in response to outputs from amplification in comparison units such that the signal from the selected switching unit having the best sensitivity amongst a number of transceiver systems is selected.
FIG. 1 shows a schematic diagram for such a multiple standard communication device 100. Typically, the multiple standard communication device comprises at least two subunits 102 and 104 each having a separate antenna 106 and 108, respectively. Examples for such a multiple communcation device are the adaption of a GPS receiver unit to a cellular mobile phone unit or the combination of two mobile phone units according to two different standards.
However, if the subunits are integrated in a multiple standard communication device, e.g., through attaching the subunits with a dedicated connector or even by building the first subunit into the housing of the second subunit problems arise due to the decreased distance between both functional units.
Further, in the sense of the present invention there exists no restriction to any specific combination of two or more subunits as long as a plurality of communcation standards are integrated into the multiple standard communication device. Thus, possible combinations could be any pair of subunits adapted to standards taken from a group consisting of GSM900, GSM1800, GSM1900, GPS, AMPS, PDC, CDMA, WCDMA, DAMPS, or positioning system standards GPS, Glonass, EGNOS, WAAS, etc.
As shown in FIG. 1, in a multiple communication device there arises the operation condition that a first subunit 102 adapted to GPS is receiving a signal while a second subunit 104 adapted to GSM is transmitting a signal. Here, in case the first subunit 102 is operating very next to the second subunit 104 the proper operation of both subunits may be prevented since large transmitter output signals from the GSM unit may degrade the reception performance, e.g., the receiver sensitivity in the GPS unit 102.
In particular this is the case if the transmission signal level at the second subunit 104 and the reception signal level at the first subunit 102 differ significantly. E.g., GPS signals are transmitted via satellites and the reception level is lying several 10 dB below, e.g., the GSM transmission level.
Therefore, to increase the receiver sensitivity there is carried out a reception signal averaging or equivalently correlation in the related subunit 102 to increase reception sensitivity. The longer the time spent for reception signal averaging the higher the sensitiviy gain will be.
However, the output of transmission signals by the second subunit 104—e.g., in a burst mode according to digital TDMA in GSM—may prevent the proper operation of the first subunit 102 since the generation of transmission signals at the antenna 108 with signal levels much higher than the reception signals at the antenna 106 may degrade the reception performance, e.g., the receiver sensitivity at the first transceiver subunit 102.
In the following a more detailed description of problems underlying the present invention will be given with respect to a multiple standard communication device where a second subunit is adapted to the GSM mobile communication standard and a first subunit is a GPS receiver for location specific services.
As will be easily understood by a person skilled in the art the problems discussed are not specific to the GSM/GPS combination but can occur as well in any other multiple standard communication device. Also, the standards underlying the GSM mobile communication system and the GPS localization system relate to technological background of the present invention and are not described in further detail here.
The GSM system being based on the digital TDMA/FDMA transmission technology is using, e.g., one out of eight time slots for the transmission of signals unless multi-slot operation is activated. Therefore, in case of co-operating of such a GSM transceiver subunit and such a GPS receiver subunit in a multiple standard communication device the GPS receiver subunit sensitivity is degraded by a factor of ⅛ or equivalently 9 dB during transmission by the GSM transceiver subunit.
This value increases when more than one time slot per frame is used for transmission.
Further, the synchronization of the GPS receiver subunit to the satellite may be lost during a long lasting data or voice transmitter signal outputted by the GSM transceiver subunit.
Still further, it may be difficult to maintain GPS receiver subunit functionality in case of emergency at any time during operation thereof.
In addition, as shown in FIG. 2 illustrating the input/output behaviour of a low noise amplifier increased interfering input signals create undesired mixing signal products and thus degrade the overall noise figure and gain of the low noise amplifier which may no longer be considered as linear device.
A parameter describing the ability to handle increased input signals and shown in FIG. 2 is the 1 dB compression point and defined as the output power level at which the gain drops by 1 dB. Typically, the noise figure and the 1 dB compression point of a low noise amplifier strongly depend on the biasing condition of the amplication element—e.g., a transistor.
Also, it is important to note that a low noise figure found at a low amplification element current usually does not correspond to a high 1 dB compression point and vice versa. This is important for battery powered transceiver subunits where a very low current consumtion is needed to achieve a sufficiently long operation/standby time. In other words, this means that the reduction of the amplification element operative current—e.g., the collector current of an amplifying transistor—per se does not lead to an improved blocking behaviour but that further steps are necessary to achieve a satisfying parallel operation of different subunits in a multiple communication device.
In view of the above, further details with respect to the multiple standard communication device adapted to the GSM mobile communication standard and having a GPS receiver for location specific services will now be discussed by way of measurement data shown in FIGS. 3 to 5.
A GPS receiver subunit must pick up very weak localization signals from a noise background. Here, the minimum received power in the L1 band for the CA code is about −160 dBW when the space satellite transmitting the signal is at two elevation angles of 5° from the user's horizon and zenith. Between these two elevation angles the minimum reveived power levels gradually increase up to 2 dB maximum, see Understanding GPS Principles and Applications, E. D. Kaplan, Artech House, 1996.
On the other hand at the same time the GPS receiver subunit is to reject a large number of much stronger unwanted signals. As already outlined above, a low noise amplifier in a GPS receiver subunit must handle interferring transmission bursts generated by the GSM transceiver subunit. The GSM 900 subunit generates a maximum output power in the range of 33 dBm and the maximum output power of the GSM 1800 and 1900 transceiver subunit is 30 dBm.
Further, the actual interferring input power according to the GSM transmission burst signal input at the GPS antenna and supplied to the low noise amplifier of the GPS receiver subunit strongly depends on the relative position of the GPS antenna to the GSM antenna.
The low noise amplifier in the GPS receiver subunit used for the measurements shown in FIGS. 3 to 5 uses a transistor of the BFP405 type as amplification element having a noise figure and power gain at different bias operations points at 1.6 GHz as listed in the following table:
|S21| * |S21| [dB]Fmin [dB](Power gain)(Noise figure)Ic [mA](VCE = 1 V, f = 1.6 GHz)1.010.70.971.513.50.972.015.31.012.516.71.073.017.71.134.019.01.225.019.91.336.020.61.428.021.31.5810.021.81.72
The impact of different bias operation points or equivalently of different collector currents Ic on the overall GPS receiver subunit will be discussed in the following.
FIG. 3 shows a low noise amplifier gain versus input signal representation in the presence of an interfering signal at 900 MHz for a low noise amplifier used in the GPS receiver subunit. From FIG. 3 it may be seen that a low noise amplifier gain compression starts at interferer levels in the range of −30 dBm to −25 dBm, depending on the collector current Ic.
Further FIG. 4 shows a noise figure of the low noise amplifier in the GPS receiver subunit in the presence of an interfering signal at 900 MHz. As shown in FIG. 4 the noise figure of the low noise amplifier deteriorates with increasing interferer level according to approximately the same rate for each collector current Ic 1.5 mA, 2.5 mA, and 4 mA, respectively.
Further, FIG. 4 shows a moderate increase in noise figure for interference levels up to −20 dBm and an increased interference level according to a factor of approximately 3 for an interference level of −10 dBm. While the illustrated results have been captured at 900 MHz the same results are to be expected for interfering signals at 1800 MHz and 1900 MHz, respectively.
Further, FIG. 5 shows the impact of a variation of noise figure and gain of the low noise amplifier of the GPS receiver subunit on the total noise figure and gain of the GPS receiver subunit. Here, the assumptions underlying the calculation of the overall noise figure are as follows: G=−3 dB, NF=3 dB for the filter of the bandpass type, G=39 dB, NF=7 dB for the amplifier and mixer gain, and gain and noise figures are assumed to be constant.
As shown in FIG. 5 the overall noise figure of the GPS receiver subunit increases generally with decreasing gain of the low noise amplifier comprised therein and with an increasing noise figure of the low noise amplifier of the GPS receiver subunit itself. Thus, to optimize performance at the GPS receiver subunit it would be necessary to maximize the gain of the low noise amplifier and minimize the related noise figure.
Heretofore it has been proposed to employ, e.g., means for filtering at the input of the low noise amplifier of the GPS receiver subunit in order to reject strong out-of-band input signals to the low noise amplifier and thus to reduce gain compression. Another option would be to increase the collector current Ic of the transistor realizing the amplification element of the low noise amplifier. However, additional filtering in front of the low noise amplifier increases the overall noise figure of the GPS receiver subunit due to the losses in the filter. Further, collector currents kept on constanly high levels are not desireable for battery powered transceiver subunits due to the decreased operation time.
While in the above a specific configuration with respect to a GPS/GSM multiple standard communication device has been described it is to be understood that any other combination where signal levels for transmission/receptions levels differ significantly between different subunits in a multiple standard communication device lead to identical technical difficulties and performance degradations.